The invention relates to a memory device for an electrical shutter for single lens reflex cameras.
A single lens reflex camera includes a movable reflecting mirror disposed at an inclination of 45.degree. between a taking lens and a film surface so as to supply the light which is transmitted through the taking lens to the light path of a finder in order to focus an image at a position conjugate to the film surface, which image is viewed through an eyepiece and Pentadachprism (pentaprism), thereby enabling the same image as the one to be taken to be viewed through the finder.
In such a camera, a proper determination of exposure factors such as shutter speed, diaphragm aperture or the like is automatically effected by conducting photometry prior to taking a picture, by means of a light receiving element located in the light path of the finder. This technique is known as TTL (through-the-lens) photometry. When an electrical shutter is applied to such a camera, because the movable reflecting mirror is retracted from the taking light path thereby closing the light path of the finder, the incidence of light on the light receiving element which is located in the light path of the finder for determining the shutter time is interrupted, which results in the difficulty that a proper exposure time cannot be determined by the electrical shutter. To avoid such difficulty, it is conventional when an electrical shutter is applied to a single lens reflex camera to store the photometric value, obtained before the movable reflecting mirror is retracted from the taking light path, in some form which enables a proper shutter time to be determined in accordance with the stored photometric value.
Both analogue and digital memory means are available for storing the photometric value. The analogue means may comprise a CdS light receiving element, and a acapacitor which stores the voltage developed across the CdS element in a form in which it is logarithmically compacted against the amount of light, the stored value being again logarithmically expanded to drive a shutter in the up-position of the mirror. The digital means performs the steps of pulse compaction, storage, expansion, retrieval and the like. The capacitor used in the analogue memory system is susceptible to self-discharge to cause an error in the stored value when it is retrieved after a prolonged period of storage time. By contrast, the digital system, while it does not suffer from such an error by virtue of its using pulses, must employ an extremely complex arrangement for the pulse compaction, storage, expansion and retrieval, which results in a very high cost.